A new gene (pLTR) has been discovered at the 3' terminus of mouse mammary tumor virus (MMTV) within the long terminal redundancy (LTR). The presence of this gene has now been confirmed in exogenous C3H-S MMTV, and in two endogenous proviruses of C3H, Mtv-1 (unit V) and unit II. A pLTR message encoding the information for this gene is uniquely expressed in preneoplastic mammary tissue (hyperplastic outgrowth, HOG) induced in Balb/c animals by the chemical carcinogen DMBA. The role of this gene in mammary carciogenesis and/or MMTV regulation is under investigation. Deletion analysis of molecular chimeras between the MMTV LTR and the v-ras transformation gene from Ha-MuSV has been performed to identify and localize the steroid target site in the MMTV LTR. The hormone regulatory sequences are located within 200 nucleotides of the MMTV cap site. NIH 3T3 cells transformed with the MMTV v-ras fusions have been established in serum-free media. The transformation phenotype of these cells is under the control of glucocorticoids; concentration dependent phenotype switching parallels the binding curve for dexamethsaone binding to the glucocorticoid receptor. This system is being exploited to select cellular mutants in the response to p21 transformation and in the hormone pathway.